Spoked to Death
Join Date: Jul 2004
Location: Boulder, CO
Bikes: Salsa La Cruz w/ Alfine 8, Specialized Fuse Pro 27.5+, Surly 1x1
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If you are doing the aluminum you should take a good look at some downhill alumnium frames, and take some measurements. If you can look at a frame that isn't built up yet, that would be perfect. Notice where they put their gussets, and plan for gussets in the same places. Also, don't be afraid to overbuild, it is a chopper after all.
Steel is generally my prefered bike material, but aluminum might be nice for such a big bike.
I'd gusset at the headtube/top tube junction, and around the bottom bracket. Perhaps behind the seat tube as well. Don't bother with butted tubing, and make sure your mitering is dead on. If you can arrange to have the entire frame heat treated after welding, that would be best, otherwise you might consider a 7xxx aluminum, which I've heard doesn't require the same amount of heat treating after welding.
And finally, post pics when you are done.